Dependence of heme accessibility in horseradish peroxidase on Ca2+
1998
Coates, A.I. | Cook, M.P. | Feezor, R. | Schuh, M.D.
The rate constant for quenching, kq, of the pulsed laser-induced phosphorescence of 6-bromo-2-naphthyl sulfate (B) was measured at room temperature to be 4.6 x 10(8) and 7.6 x 10(8) M(-1) s(-1) for quenching by horseradish peroxidase (HRP) with and without bound Ca2+ (CaD-HRP), respectively. Quenching of B phosphorescence by apo-HRP was found to be biexponential and gave evidence that quenching by HRP and CaD-HRP occurs predominantly without the formation of a bound complex of 3B and protein. It was also concluded that quenching occurs predominantly after B migrates through the single substrate channel into the vicinity of the heme. Since the activation energy for quenching was found to be insensitive to the presence of Ca2+, the conformation and dynamical motion within the substrate channel were concluded to change little between CaD-HRP and HRP. It was concluded that if enzyme enhancement induced by Ca2+ is due to structural stabilization of HRP, the stabilizing influence is not transmitted strongly into the substrate channel.
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